Engineer Design Test 4 (EDT-4) was conducted using a prototype YAH-64 aircraft. This test was conducted at Palomar Airport, Carlsbad, California, (elevation 328 feet) between 10 and 29 November 1980. Approximately 33 hours were flown during 27 flights. Major changes affecting performance and handling qualities were made to the YAH-64 since the last evaluation (EDT-2). These included a new, digital stability augmentation system, and a redesigned empennage featuring an automatically programmed stabilator and an increased diameter tail rotor. In previous evaluations the flight envelope and scope of test were limited by structural loads and continuous monitoring of those loads was required for all flights. During this test, the flight envelope was much larger and no structural limits were encountered. Hover and level flight performance of the aircraft have been improved since EDT-2. Handling qualities have been greatly improved. The most significant areas of handling qualities improvements were in low-speed flight characteristics, including directional control margins, short-period dynamic stability characteristics (particularly in high speed flight), and in aircraft pitch attitude during approaches and IRP climbs. Three deficiencies were found during this evaluation. The disengagement of the HARS, DASE, and automatic operation of the stabilator, and erroneous activation of the engine out/low rotor speed audio warning tone with failure of the No. 1 generator random failure of the master caution light to illuminate with the illumination of some caution on warning panel segment lights; and the restricted pilot's field of view caused by canopy frame structure during NOE and contour flight.